In recent years, a multimedia server system has attracted a lot of attention, in which multimedia data including various kinds of data (e.g., image, voice, character) are stored in a mass storage connected to a computer, and the stored multimedia data are transmitted through a network to terminal units.
FIG. 13 is a block diagram showing an example of a multimedia server system for sending a motion picture stored in a magnetic disk to a personal computer (hereinafter referred to as "PC") connected to a network (refer to "Video server adapted to motion pictures by expanding NFS and TCP/IP", Nikkei Electronics, Sep. 25, 1996, p133-p141).
In FIG. 13, reference numeral 22 designates a server, 23 designates a disk array unit comprising a plurality of magnetic disks, 20 designates PCs, and 21 designates a network transfer unit.
The multimedia server system so constructed operates as follows.
Initially, each PC 20 sends a request to read data of a motion picture through the network transfer unit 21 to the server 22. Receiving the requests to read motion picture data from plural PCs 20, the server 22 reads the motion picture data corresponding to the respective requests from the disk array unit 23 at a uniform rate required for continuous reproduction of motion pictures. Then, the server 22 transmits the read data at a uniform rate through the network transfer unit 21 to the respective PCs. Receiving the motion picture data, each PC 20 displays the motion picture on a screen using software on the PC 20 or hardware.
For example, the reference literature mentioned above discloses a specific system for reproducing arbitrary motion picture data stored in the disk array unit 23, simultaneously on forty PCs 20 at a rate of 1.5 Mbit/sec.
Since the disk array unit 23 is very expensive, in order to reduce the cost of the whole system, a multimedia library unit, such as an optical disk library, may be connected to the disk array unit 23 as an auxiliary storage unit (refer to "Invitation to latest HSM technology `nessy`, No. 2, Virtual Resource and Offline Management", Monthly Sun World, October, 1995).
In this system, data of high access frequency are stored in the high-speed and expensive magnetic disk array unit while data of low access frequency are stored in the low-speed and inexpensive optical disk library. When referring to the data stored in the optical disk library, the data are once copied to the magnetic disk array unit and then output.
In this system, however, since the motion picture data must be once copied from the optical disk library to the magnetic disk, it takes a lot of time from receiving a request to read motion picture data stored in the optical disk library to start reading of the motion picture data. In case of a motion picture server, the possibility that the motion picture data copied from the optical disk library to the magnetic disk are accessed again is not always high.
As a measure to solve the problem mentioned above, instead of the magnetic disk, the optical disk library may be connected to the server. In this case, however, since an optical disk access unit (disk drive) contained in the optical disk library has a data reading rate lower than that of the magnetic disk, the number of motion picture data simultaneously and continuously read from a single optical disk is limited. Hence, when a plurality of accesses are concentrated to motion picture data stored in a single optical disk, reproduction of motion pictures is interrupted.
Furthermore, an optical disk library unit contains less number of optical disk access units than the number of optical disks contained therein. For example, an optical disk library having a capacity of 100 optical disks contains 4 optical disk access units. So, when receiving requests for simultaneous accesses to optical disks exceeding the number of the optical disk access units, these requests arc processed by time division while changing the optical disks using a disk changer contained in the library. However, it is impossible to continuously read all of the requested motion picture data at a uniform rate. Accordingly, when plural accesses are concentrated to a single optical disk library unit, reproduction of motion pictures is interrupted as well.
The above-mentioned problem also occurs when a tape library serving as a multimedia library unit is connected to the server, which library contains plural video tapes in which multimedia titles such as movies are recorded and reads arbitrary ones of these video tapes after mounting them on plural tape drives using a tape changer.